Surface densification of machine components made by powder metallurgy

ABSTRACT

A sintered powder metal part is surface densified by surface heating followed by repressing. Surface heating is preferably done to a temperature which is just below the critical temperature where the steel alloy material of the part transforms from a ferritic to an austenitic microstructure. Repressing is in a die set which is smaller than the part by 10% of the surface heated depth. The hot skin is compressed and densified between the die and the cooler, and therefore less malleable core of the part Following surface densification, the part may be resintered and/or heat treated.

CROSS-REFERENCE TO RELATED APPLICATION

This claims the benefit of U.S. Provisional Patent Applications No.60/028,415 filed Oct. 15, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to powder metallurgy, and in particular to amethod of making a surface densified and hardened powder metal part.

2. Discussion of the Prior Art

Many components of machines are required to function in rolling andrubbing contact at high surface pressures. This action causes wear by avariety of well known mechanisms including: abrasion, scuffing (galling)and surface pitting (rolling contact fatigue). An economical andwell-known method of manufacture of machine components is by powdermetallurgy (P/M) which involves compaction of a blend of fine powders ofappropriate composition in a set of tools that result in a preciseshaped preform. The preform is then subjected to heat in a processcalled"sintering" which bonds the powder particles together and alloysthe blend ingredients to form the desired microstructure. The sinteredproduct can then be heat treated conventionally to harden the surface orwhole part to increase wear resistance.

It has been found by experiment that wear resistance against heavyrolling contact requires a surface layer or skin of high integrity towithstand the subsurface micro scale cracks. These cracks eventuallypropagate and join together until a small fragment of surface materialbreaks away leaving a small pit. This process spreads to form largerareas of pitting. Eventually the machine operating noise becomes aproblem or in extreme cases, the surface fails leading to mechanicalbreakdown of the machine. In order to raise the surface integrity of aP/M mechanical component such as a roller or gear or sprocket, thesurface density must be increased to an appropriate level. This can beaccomplished in several ways including raising the density by repressingthe whole sintered product in the cold or heated condition.Alternatively the surface can be densified locally by a rolling action.In the case of a gear or sprocket, the latter involves rolling andmeshing against a master gear or sprocket at higher pressure. Thisprocess requires an expensive precision master former which has limitedlife due to wear, and depth of densification is limited.

SUMMARY OF THE INVENTION

The invention provides a method of making a surface densified powdermetal part in which, after initial compressing and sintering, the cooledpart is surface heated to a surface heated depth so as to produce a hotskin which is at a temperature above the core temperature of the part.The part is then repressed in a second die set. This compresses the hotskin between the die and the cooler, less malleable core of the part, todensify the surface of the part.

Following surface densification in this manner, the part may beresintered and/or heat treated or hardened.

Preferably, the surface heating is done to a temperature which is justbelow the critical temperature, which is the temperature at which thesteel material of the part transforms from a ferritic to an austeniticmicrostructure.

In addition, the second die set, in which the part is repressed, isadvantageously smaller in at least one dimension than the surface heatedpart by approximately 10% of the surface heated depth, to provide adesirable degree of surface densification.

These and other objects and advantages of the invention will be apparentfrom the detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention teaches an alternative less costly and potentiallydeeper skin approach to surface densification which involves localizedsurface heating of the P/M component being treated, followed bycompression of the hot surface by pressing the component into a shapeddie which is slightly smaller (e.g., in diameter) than the product. Thiscauses compressive forces in the hot surface. The unheated core acts asa restraint against which the hot surface is compressed. This is incontrast to conventional hot repressing in which the whole body ispre-heated and the hot core also compresses, thereby preventingeffective surface densification.

In the present invention, the controlled surface heating can be achievedby induction heating using a conventional high frequency industrialunit. Careful selection of power, time of current passage and cyclicfrequency enables a controlled depth of material (skin) to be heated tothe desired temperature.

One aspect of the invention involves heating the surface of the P/Msteel component to a temperature which is just below the criticaltemperature (where a steel transforms from a ferritic to an austeniticmicrostructure). This takes advantage of the lower flow stressassociated with the ferritic form. This also limits the temperature to alevel that is not too injurious to the repressing tooling. A typicaltemperature is in the region of 600 to 800 degrees Celsius and isselected based upon the steel composition, product geometry andoperations stress levels to be borne.

An example of a product which can advantageously be made in accordancewith the present invention is a transmission sprocket for an automobile.The outer teeth are used to transmit engine power to the drive systemvia a linked chain The chain links rub and roll against the sprocketteeth resulting in highly localized stresses which lead to surfacepitting as described above. The P/M process involves compaction of asteel powder blend consisting of a prealloyed base iron containing twopercent nickel, plus one percent of graphite and half a percent of apressing lubricant which is an organic stearate. The powder is blendedfor 30 minutes to homogenize the composition. It is then left to standand settle for one hour before being charged into a hopper that feedsthe powder into a compaction press die set and tooling. The compactionpress then compresses the powder, forming a compact which is ejectedfrom the tooling. The compact is then subjected to the thermal processcalled sintering (described above) which results in a structuralcomponent, after cooling to room temperature, with a density of 90% offully dense steel. The component is then subjected to surface heating byinduced currents which raise a surface layer of about 2 millimetersdepth to a temperature of 700 to 750 degrees Celsius. The part isimmediately pushed into a second die set which is preheated to about 400degrees Celsius and is smaller in radius than the surface heated productby approximately 10% of the surface heated depth, which in this case is0.2 millimeters. Therefore, the die diameter is approximately 0.4millimeters smaller than the surface heated product. This die diametermay require fine tuning for optimal results in specific cases.

Following the surface densification, the surface layer is above 95% oftheoretical density to a depth of at least 0.5 mm and preferably to 1 mmdepth, which is below the depth of Hertzian stresses in the examplechosen.

To raise the hardness of the densified surface layer, a post-treatmentof conventional induction heating and quenching is used followed bytempering to enhance teeth toughness at 180 degrees Celsius for onehour.

In another example of the invention the product is a helical gear, alsoused to transmit power in a machine. In this case the powder blend isbased upon a prealloyed 2% nickel, 0.5% molybdenum steel powder withelemental additional of 1% each of nickel and of copper powder. Theblend is completed by 0.9% graphite powder and 0.3% of organic stearate.The compaction process involves rotating tooling to comply with thehelical gear tooth form. The die walls are lubricated with a sprayedcoating of a solution of water and organic stearate. The die ispreheated to about 400 degrees Celsius so that thermal shock isminimized and the lubricant spray flash dries on contact with thesurface. The compacted preform is then sintered at a low temperature(1600 degrees Celsius) to avoid any hardening from martensite formation.The helical gear is induction surface heated to produce a 2 millimeterhot skin at 700-750 degrees Celsius and is repressed in an undersize dieas described in the first example. The resultant product is thenre-sintered in a specially modified furnace which heats the part to 1130degrees Celsius for 15-30 minutes and then fast cools to roomtemperature to produce the hardened microstructure of martensite. Thegear is then tempered for one hour at 180 degrees Celsius to completethe process. The resultant gear has a densified hard skin which isbetween 0.5 and 1 millimeter deep to at least 95% of theoreticaldensity.

In sum, the invention provides a process and resulting ferrous powdermetallurgy product which has a densified skin produced by heating asurface layer to soften it in readiness for a repressing operation in anundersized die which compresses the hot, soft skin against therelatively cold, hard core, leading to localized skin densification.This is especially useful where the component is a power transmittingpart such as a gear or sprocket or roller. A preferred method of heatingthe skin prior to repressing is by induction heating. It is also usefulto make the base material an air hardening steel which hardens duringsubsequent processing in a fast-cool furnace.

Many modifications and variations to the preferred embodiments describedwill be apparent to those skilled in the art. Therefore, the inventionshould not be limited to the preferred embodiments described, but shouldbe define by the claims which follow.

We claim:
 1. A method of making a surface densified powder metal part,comprising the steps of:compressing a powder metal material in a firstdie set so as to form a compact in the general size and shape of thepart; sintering said compact to make said part; surface heating saidpart to a surface heated depth so as to produce a hot skin which is at atemperature which is below a critical temperature of said material butabove a temperature of a core of said part, said core being below saidhot skin, said critical temperature being the temperature at which saidmaterial transforms from a ferritic to an austenitic microstructure; andrepressing said surface heated part in a second die set.
 2. The methodof claim 1, further comprising the step of heat treating said part aftersaid repressing step.
 3. The method of claim 2, wherein said heattreating comprises heating, quenching and tempering said part.
 4. Themethod of claim 1, wherein said surface heating is accomplished byinduction heating.
 5. The method of claim 4, wherein said repressingstep immediately follows said surface heating step.
 6. The method ofclaim 5, wherein said surface heating is done to a depth ofapproximately 2 millimeters.
 7. The method of claim 5, wherein saidsecond die set is smaller than said surface heated part by approximately10% of said surface heated depth.
 8. The method of claim 1, wherein saidsurface heating is done to a temperature of approximately 600 to 800degrees Celsius.
 9. The method of claim 1, wherein said powder metalmaterial is a steel powder blend.
 10. The method of claim 1, furthercomprising the step of resintering said part after said repressing step.11. The method of claim 10, wherein said resintering step includesheating said part followed by fast air cooling said part.
 12. The methodof claim 11, wherein said powder metal material is an air hardeningsteel.
 13. The method of claim 11, further comprising the step oftempering said part after said resintering step.
 14. A product made bythe method of claim 1.